The invention relates to a high-pressure three-phase gas discharge lamp containing noble gas, mercury, sodium and/or metal-halide additives.
In the high-pressure, doped gas discharge lamps known and used so far the light of an arc discharge produced, between two cylindrically symmetrical electrodes fed from a single-phase supply network is utilized. The disadvantages of a lamp of this kind result from light modulation arc instabilities (e.g. convective side deflection of the arc in metal-halide and mercury vapour lamps), further from the reignition problems occuring at zero transition of current. The stroboscope effect resulting due to light modulation of single-phase gas discharge lamps is of special disadvantage when moving objects are to be illuminated and moving pictures or video records are taken.
To eliminate these drawbacks, gas discharge lamps fed from a three-phase supply, with three electrodes arranged in their discharge space are known from various publications.
UK patent specification GB 616404 discloses a heavy-duty three-phase lamp with a spherical quartz envelope filled with noble-gas, mercury and cadmium and/or zinc. The electrodes are accommodated in the envelope so that their heads "see each other", the heads being seated in the vertices of an imaginary equilateral triangle inscribed into said envelope.
German (laid open) Patent Application DE-OS 2 542 133 a three-phase discharge lamp, but designed to provide a discharge of linear pattern between the electrodes of the discharge vessel.
PCT International Application No. WO 83/041140 discloses a three phase discharge lamp including three electrodes accommodated in a discharge vessel having a very special shape and, in addition, a fourth electrode connected to the star point is built into the discharge space.
In East German Patent Application No. DD 215423, a low-pressure discharge lamp containing a layer of a luminophor is described, the discharge vessel being confined by planar surfaces and also having a fourth electrode connected to the star point of the three-phase electrode system.
In Japanese application JP 56-099965 (Kokai) a discharge lamp is described in which the three heated electrodes are located in a glass discharge vessel enclosed by planar lateral surfaces, but the discharge itself is also of linear shape.
All these lamps, however, have failed to find practical use, either because of their poor colour rendition (GB 614404), or because of the complicated shape of their discharge vessel or the requirement of adding a fourth electrode (WO 83/041140, DD 215423), or because, in spite of accommodating three electrodes in the discharge space, a linear discharge (DE 2542133, JP 56-099965A), instead of a spherical pattern, is provided by them.
A long-standing demand prevails, however, on the part of consumers of a high-pressure discharge lamp that is capable of providing illumination without giving rise to disturbing stroboscopic effects but yet has satisfactory colour rendition. The discharge lamps described in the patent specifications mentioned above fail to fulfil these requirements.
On the other hand, it has been recognized that the way to obtain a quasi-stationary spheroid plasma is by bringing about a spherical discharge in a discharge space filled with a substance ensuring good colour rendition, and where the time constants of the processes taking place are selected with a view to achieve this aim. It is important to make the discharge itself spherical rather than some rotating linear shape, because this is the only way of obtaining the required light source free of producing the undesired stroboscopic effects.
Further, by using a ceramic discharge vessel the possibility of setting up a three-phase, high-pressure sodium lamp containing a spheroid discharge plasma exists by properly arranging in the discharge space the three electrodes each fed from a single phase of the supply source. By proper arrangement it is meant that the electrode heads "see each other" and are sited in the vertices of an equilateral triangle inscribed in the discharge vessel.